text:
Why would a pilot on final to a towered airport not speak up when ATC clears another aircraft to land on the same runway, without mentioning them?
Every time I watch AOPA's Accident Case Study video Communication Breakdown, I'm left wondering. Here's an aircraft (in this case, '1DA) on final approach for landing on a towered airport, when another aircraft ('4SR) is cleared to enter the traffic pattern, and gets cleared to land before they turn base, without ATC mentioning the aircraft already on final. One could make an argument that the pilot of '4SR should be aware of '1DA, but somehow, they apparently weren't. Once the pilot of '4SR is made aware of the traffic already on final, they make what I can only describe as a hasty attempt at an evasive maneuver which results in '4SR crashing into the ground, killing everyone aboard '4SR.
Yes, this is a towered, controlled airport; ATC should be the one to inform the incoming aircraft about the one on final. But for some (here unimportant) reason, ATC fails to do so.
To make matters worse, '1DA is flown by a student pilot and instructor, so there's two people there, either of whom (and certainly the instructor) can use the radio.
In such a situation, why wouldn't the pilot of '1DA speak up?
'1DA was still some distance out from the airport. It seems to me that besides the obvious of maintaining situational awareness, there were several rather obvious options available here:

No later than when the pilot reported being on base, ATC could (maybe should) have instructed the pilot of '4SR to make a right turn to re-enter the downwind leg, and to then extend downwind and land behind '1DA. Yes, reentering the downwind leg from the inside would be unusual, but it seems as though it would have kept them well out of the path of '1DA on final.
ATC could have told '1DA to go around when the controller realized that '4SR was unaware of '1DA, allowing '4SR to land ahead of '1DA; things could be sorted out once both where on the ground (the video mentions this possibility).
The pilot of '1DA could have made a position report specifying being number one on final for the same runway, when ATC didn't mention them to '4SR.
The pilot of '1DA could have aborted their landing and executed a missed approach, announcing this, effectively letting '4SR land ahead of them.

Any of those (and probably a few other) options seem like it could have prevented the accident altogether, at the cost of a relatively minor inconvenience to either '4SR or '1DA. So why would no one do anything until it was (obviously in retrospect in this particular example case) too late?

I've been cleared to land (without saying "follow XYZ") while another aircraft was on final. It is expected that the aircraft will still follow the standard pattern which if 4SR had done, would have put them behind 1DA. Instead 4SR understood that as "land now" and turned immediately to base, which was very wrong. I can't listen to it now, but the controller should not have said (if I remember right): "Cut it in, cut it in tight for [rwy]". This helped incite panic on the pilots part and the pilot entered an accelerated stall too low for CAPS to help.

The crew of '1DA did not realise something was amiss any sooner than the controller did, when '4SR reported base. At that moment, it is still better to give the controller chance to react first as they usually have best idea of the overall situation. Meanwhile, '1DA was making sure they see '4SR as maintaining visual separation was their safest fallback solution (and they did see them as they confirm to tower they saw the '4SR's fatal manoeuvre).

why wouldn't the pilot of '1DA speak up?

The simple answer is that you are not trained to "speak up" at a towered airport, you are trained to follow instructions from the tower and there is an assumption the tower will provide separation and appropriate clearances. As an ultimate fall back you are always supposed to "see and avoid".
For the official report you should reference the NTSB report but if you listen to the ATC audio in the linked video as well as their explanation of the link the pilots had no reason to think there was an issue and the clearances given were rushed but explicit. As I note above AOPA also notes

"it is a pilots responsibility to visually confirm final approach"

You can also hear that the pilot of '4SR was giving confusing instructions about base extension and clearance to land. The pilots was not incorrect in his approach actions but made a critical go-around error and may have been a low altitude, dirty config, stall.
Ultimately the lower aircraft on approach had the right of way so '1DA had no reason to speak up as they were in the right. AOPA states that the pilots may have thought "he was overtaking the aircraft from above" which is incorrect, or at least procedurally incorrect as '4SR would have had to yield to '1DA anyway.
In the end the controller should have simply issued

'4SR Go Around

Any time, you as a pilot hear that you should build the habit to just go around. As PIC you can ALWAYS execute a go around, controllers should be prepared for it and more importantly you as the PIC should be prepared for it.

Indeed, telling '4SR to go around is another option which I failed to include in my list.

It was reasonable for 1DA to assume 4SR would fly a standard pattern, but Tower not explicitly telling 4SR they were #2 and/or confirming they had traffic in sight would have made me nervous enough to say something. Perhaps that's because my home airport is very busy and Tower always does both: they will not clear you to land until you report all preceding traffic in sight OR they call your base on extended downwind (the norm) when you pass abreast of the plane ahead of you on final. This sort of accident is exactly why they do that even though VFR rules don't require it.

It's the controller's responsibility to issue instructions and a pilot's responsibility to follow them unless doing so is unsafe. The pilot of 1DA didn't speak up because there was no reason to at that point; he didn't know of 4SR's intentions and probably thought that 4SR was going to fly a normal pattern just like the controller did. When he saw that 4SR was cutting the corner (obviously in  a hurry, never a good thing) in front of him the instructor pilot of 1DA took control from the student and started a go around.
Going around is a busy time, you have to change pitch and power while maintaining airspeed and keeping the airplane in balance. The rule is Aviate, Navigate, Communicate, you don't make any radio communications until you have an established and controlled climb. 1DA would have made a going around call once that was done, but in any case the controller was, as the NTSB report says, "yelling" at 4SR so he wouldn't have been able to report anyway. 1DA wouldn't have reported number 1 because it's up to ATC to say who's #1, #2, etc.
4SR had multiple risk factors. According to the NTSB report it was 22 pounds under the weight limit (which incidentally meant it would have been over the limit on departure), and probably loaded with a C of G at or behind the rear limit. The pilot had elected to do a "slam dunk", close in approach rather than the normal pattern (probably intending to land long so there was less of a roll-out), was at a high approach speed and likely in a rush, with get-there-itis. The high weight would have increased stall speed and a rearward center of gravity would have decreased stability and control authority. The pilot jammed full throttle on quickly when at the limits of performance and couldn't maintain control.

In the video, it's clearly tower instructing 1DA to go around after 4SR goes into the spin. Did you notice any indication they went around on their own already?

In the NTSB report the pilot of 1DA says he had already initiated a go-around before 4SR went into a spin @JanHudec

Ah, ok, then it makes sense.


text:
How to handle seeing someone online claiming to be a pilot or pilot in training, who one feels shouldn't be flying, in the EU?
Suppose that on some aviation-related Internet discussion forum, one comes across contributions from a user who claims to either be training for a pilot's license, or to have a pilot's license.
However, the nature of their contributions are such as to call into reasonable doubt whether that person should be flying at all, or at least flying without an instructor closely monitoring from the next seat over in the cockpit.
Not being a forum administrator, there is no way for the person seeing these contributions from being able to access detailed records of the user's activity, such as IP addresses they are browsing from or e-mail addresses associated with their account. However, it stands to reason that the forum records these details.
The user making the claims says to be within the EU, and for simplicity's sake let's assume that the user noticing the claims is within the EU, but the two are not necessarily within the same EU country, so it's not as easy as just contacting one's national authority and be done with it. The forum itself may be hosted outside of the EU, but with a worldwide audience.
If one wants to, for the sake of aviation safety, bring such a situation to the attention of the competent authorities to be looked into, then how would one go about doing so?

How does one determine which authorities to contact? For example, which country's authorities should be contacted initially, and how to locate the proper authority/authorities within that country?
Who (which function) at those authorities should/does one likely interact with? (Basically, if hypothetically calling their switchboard phone number, what to ask for?)
What information is likely to be needed in an initial report, so that one can compile it prior to contacting the authorities in question?

If someone knows of better tags for this, do feel free to retag.

And, if someone sees some way this question can be improved, feel free to mention it or propose an edit...

This particular site is for "aircraft pilots, mechanics, and enthusiasts", and is free.  It would be wise to cross check any information obtained here.

This is borderline off-topic for me, trolls and posers are common on internet sites, and how you deal with them is opinion based.

@GdD How to deal with trolls and posers is probably opinion-based, as you say, but how to take one specific action seems like an answerable question to me. It might possibly still be off topic (though I think it fits within this site's scope, mostly as navigating the bureaucracy surrounding aviation, or I wouldn't have posted it here in the first place), but I don't think this should be off topic specifically as being primarily opinion-based (which would basically mean that a fact-based answer isn't possible).

In general, one wouldn't.
If the user in question has never touched the yoke, there is no issue - it's not illegal to lie on the internet.
If they're in training, it's on their instructor to set them right.
This leaves the case where they do hold a current license. Presumably you want to notify the local aviation authority that they might not satisfy its requirements.
Now put yourself in the local FAA-equivalent's shoes. Supposedly a certified pilot has posted on the internet that the artificial horizon is a lie and one should always trust their gut over it. What recourse, exactly, do you have against them? They're not under any obligation to teach flying to the general public, and there's no regulations or prescribed penalties for publishing incorrect information on the internet.
NB: It would be very different had they identified themselves as a commercial pilot for a specific carrier. In that case, they'd be presenting their airline in a bad light, and the freedom of employment (in the US, varies in the EU) would allow their employer to take action to protect their reputation.
A PPL, OTOH, is a more exclusive cousin to the driver's license, and merely indicates that its holder has at some point convinced their instructor that they were capable of flying at that moment, while meeting a number of formal requirements, and has renewed it since at required intervals.
The forum's administration might want to take action to keep false information away from their board, if you inform them, but interaction with them is a subject far removed from aviation.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

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How to calculate lift and drag coefficients of an airfoil with slotted flaps?
I am doing a project attempting to determine the increase in lift and drag coefficients of an airfoil as the angle of its flap increases. In my research, I have found the thin airfoil with flap approximation in Mechanics of Flight:

However, this is a plain flap and not one for a slotted flap.
Similarly, I basically cannot find anything on formulas that determine the drag coefficient with a flap deflection.
Of course, I am aware of the DATCOM methods, but I will need something physics-based.
Any help will be appreciated.

Slotted flaps work quite like camber flaps, only that the slot allows to create a new, fresh boundary layer on the flap surface which delays flow separation. Therefore, for small deflection angles the approximations for camber flaps can be applied to slotted flaps, too. If the flap deflection also in involves an increase in wing area by a backward movement of the flap, correct the result by the relative increase in chord.
For a very coarse drag approximation you may add a drag coefficient increment for every degree of flap extension as shown in NASA Contractor Report 4746, which is the source for the picture below:

The detailed drag increase depends a lot on the size and shape of the slot and should be looked up in the literature of the particular airfoil.
For higher deflection angles you will need to employ formulas which include viscous effects. The simplest ones are already far more complex than your one-liner; here it is best to use a panel code like MSES by Mark Drela. This will give good values for both lift and drag.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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Could entrained air be used to increase rocket efficiency, like a bypass fan?
Could entrained air flow could be used to increase thrust in the similar way to how a bypass fan works? - The total thrust is √(2)
where  is the total exhaust effective mass flow straight backwards, and
is the power input, from fuel. If the mass rate can be increased as with a bypass fan, the thrust can increased. This could significantly reduce fuel required in early stage, where fuel use is greatest. The effective exhaust speed is reduced, but the increase in mass flow more than compensates.

It's called an air-augmented rocket.
They add a lot of weight, but yes, they work.
They've been discussed on SE here: Limits of bypass ratio in air-augmented (ejector-jet/ducted) rocket

thanks, not a completely insane idea then.

Yes
At least, the investors of the Skylon project seem to think it's a good idea. Time will tell whether they were brilliant visionaries or rich people lighting a massive bonfire with money.
Not quite an "air-breathing rocket" but perhaps of interest anyway is the Star Raker design by Rockwell. It's relevant because the idea of using the atmosphere as a free reaction mass + oxidizer is the key working concept, IMO. Star Raker just put the "jet" and "rocket" portions of operations into separate components, rather than trying to combine them like Skylon does.

Fascinating, thanks for the links.

While the idea is attractive on paper, adding significant complexity to a rocket isn’t good for reliability, although just making the rocket bigger is straightforward within some practical limits.  In the context is a rocket launch, the cost of a couple of tonnes of extra fuel is probably peanuts.


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twist correction of fan blades
How can the blade's angle be effectively adjusted to meet prescribed tolerances during shop repairs?
I am conducting a research paper on aircraft engine blade maintenance, and I have encountered limited information on the repair methods and techniques used to correct the twist angle.

Maybe because there isn't a need to "correct" the twist angle?  Most blades are blended to remove knicks, and if damage is beyond a certain point they are simply replaced.  I have never heard of one untwisting from normal use.

Isn't twist angle pretty well set when the blade is cast / manufactured? Wouldn't it get destroyed along the way to changing that angle while in service?

Here is a way to grasp the subject.
Imagine that with accumulated flight hours, the blades of (for example) the power turbine get asymmetrically eroded, resulting in a change in the effective "twist angle" of the blade and a reduction in its efficiency.
For this to result in, say, a 10% reduction in the power generated by the turbine disc would necessarily involve a significant thinning of the blades themselves, rendering them unserviceable and requiring their replacement.
This means that the blades would not get reworked to correct the twist angle at that point, because they would be beyond their thickness wear limits.


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Is there a simple formula for estimating supercharger throttling losses?
I’ve noticed that in both Aerodynamics for Naval Aviators, and most old WWII engine power charts, power vs altitude seems to be composed almost entirely of linear relationships: at a given supercharger speed, (assuming a single stage supercharger with no aftercooler, Water Injection, or other additional power boosting system active), power from sea level increases in a linear fashion until critical altitude, and from there, power decreases at a linear rate
I already know the basics of why power increases, then decreases: the engine must be throttled below critical altitude to prevent overboost, and this results in a loss of efficiency in accordance to the amount of throttling. And above critical altitude, the supercharger simply cannot provide enough of an increase to pressure to achieve the desired manifold pressure
I also understand that roughly, 1,000ft increase in altitude will correspond to a 1”HG decrease in air pressure, so that gives me some clues to work out the relationship above critical altitude. But I don’t understand how to determine how much power a given supercharger speed gives BELOW Critical Altitude, where Max manifold pressure is available, but must be throttled

I'm not sure if this is exactly what you are looking for, but may be of help:
A Journal article "Supercharged Engine Performance, Calculated and Actual" (see link below) has information that to me seems to match your question. Table on page 261 and a graph on page 263 present calculated and actual power outputs of a supercharged aviation engine in different power settings (named "speed"). As per its title, the article focuses on differences between calculated and actual performance, but does so by reviewing data on different power settings (and configurations) and should thus provide information about throttling losses.
The full document is downloadable from the page http://www.jstor.org/stable/44434046
Source:
Chenoweth, O. (1927). Supercharged Engine Performance, Calculated and Actual. SAE Transactions, 22, 256–268.


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What's the thrust of a given propeller at zero airspeed?
If memory serves correctly, propeller thrust for a given propulsive horsepower is inversely proportional to airspeed, assuming features like constant RPM prop, and being outside Mach Effects
The problem is, such a relation falls apart at zero airspeed by virtue of dividing by zero, and even before that, it would predict thrust sufficient to rip the propeller blades clean off the aircraft.
Obviously, there must be something more to this situation, but I don't know what the rules are in that. How do we know the thrust of an ideal propeller at zero airspeed if we know the thrust for given power at some higher airspeed?

Peter's answer and its comments may enlighten.

Does this answer your question? How can I calculate the thrust of a propeller?

Should be relevant: What is the equation for calculating static thrust?

The trick to avoid a division by zero is to use the speed at the plane of the propeller disk and not the speed at infinite distance. Then you can use the same equation for a moving and a static propeller.
Air of density $\rho$ flows through the propeller disc of diameter $d_P$, creating a thrust force $T$ by being accelerated by $\Delta v$. The air speed ahead is $v_0 = v_{\infty}$ and the air speed aft of the propeller is $v_1 = v_0 + \Delta v$. In the plane of the propeller it can be assumed that air speed is $v_{\infty} + \frac{\Delta v}{2}$:
$$T = \pi \cdot\frac{d_P^2}{4}\cdot \rho \cdot \left( v_{\infty} + \frac{\Delta v}{2} \right) \cdot \Delta v$$
In the static case $v_{\infty}$ is zero:
$$T = \pi \cdot\frac{d_P^2}{4}\cdot \rho \cdot \frac{\left(\Delta v\right)^2}{2}$$
Without knowing $\Delta v$ it is impossible to solve. Here the static thrust equation which uses engine power $P$ helps:
$$T_0 = \frac{P\cdot\eta_{Prop}}{\sqrt{\frac{2\cdot T_0}{\pi\cdot d_P^2\cdot\rho}}} = \sqrt[\LARGE{3\:}]{P^2\cdot\eta_{Prop}^2\cdot\pi\cdot \frac{d_P^2}{2}\cdot\rho}$$
Still, for a solution you need to know the propeller efficiency $\eta_{Prop}$. For the static case this parameter can at most become 0.5. For practical results use 0.4 with constant speed props and - depending on propeller pitch - for fixed-pitch propellers lower values if the propeller is optimized for higher speeds.

the relation falls apart at zero airspeed by virtue of dividing by zero

With props, thrust is affected by airspeed, but it is important to realize engine power output only works against the drag of spinning the propeller (and of course its own internal friction).

Engine Brake Horsepower = prop Drag torque/time

The thrust output of the propeller depends on the amount of air mass accelerated and it's angle of attack to the relative wind and its coefficient of lift (as an airfoil).

this is known as prop efficiency

Props are a bit more of an art because, while they lose efficiency as airspeed increases from an air mass acceleration view, efficiency greatly increases if the prop blade angle of attack is properly aligned with the relative wind.
Prop propulsors generate thrust far more efficiently as spinning airfoils than they do as mass movers.  This is why variable pitch prop blades enable much higher airspeeds.
In summary, the power output of an engine is only related to its fuel burn.  The load on the engine is prop drag.  No need to lose your mind with division by zero.

thrust is derived by multiplying in the efficiency factor, which can be positive, zero, or negative (reverse thrust)

Finally, it is important to consider whether or not the given situation is transient or steady state.  Unless brakes are applied, an aircraft generating thrust will begin accelerating until thrust = drag.


text:
To what extent is LLF forecast of icing conditions a concern in VMC?
First off, I know that flying into actual icing conditions in an aircraft not equipped to handle icing is a Very Bad Idea ™ for any number of reasons. That's not what I am asking about here.
Rather, the situation I am asking about is where (for example) the low-level forecast indicates a possibility of or the existence of (particularly, light or moderate) icing conditions, while visual meteorological conditions prevail throughout the area of one's flight, and there are few or no clouds (scattered or clear) nearby.
In such conditions, is icing an actual concern?

Well if you have a forecast of icing, and you go outside and the sky is clear, the forecast is wrong no?
In any case, when you are flying around VFR and there is the possibility of icing on your route, your main concern is the freezing level, and whether you can stay well below it in the event you encounter rain or drizzle or whatever (aside from visibility requirements of course).  If you will find yourself close to or above the freezing level where you plan to cruise, you have to avoid visible moisture, period.
You're VFR so you will be avoiding cloud anyway, but if there is anything coming down out of the clouds you will pass through, you need to be a couple thousand feet below the freezing level to be safe.
So, if you're going from a to b, at 5000 ft, and the freezing level is 8000 ft, and there is light rain forecast with icing expected, say between 8000 and 15000ft, you're fine at 5000 ft IF, the freezing level is going to stay well above you on your route, but you need to be confident that temperatures aren't going to drop and force you lower part way there.
If the sky is clear or scattered, nice VFR in other words, you are unlikely to see icing in a forecast anyway.

I have seen moderate icing mentioned in the LLF, yet been able to look out the window and barely see a cloud anywhere. The problem here is that even the relevant portion of the LLF covers a pretty large geographical area and IIRC up to FL100, far from all of it relevant to any one flight, and it seems to me that at least on occasion, someone has been hedging their bets by mentioning the possibility of moderate icing conditions (our LLF doesn't cover light icing at all) in what seems to be a larger than required area. My theory books offered little in terms of guidance, hence my question here.

Also (but not exclusively) given your mention of the freezing level, if the aircraft is equipped with an OAT sensor, keeping that well above freezing should also help, no? (Assuming for a second that it works properly...)

Yes you need to monitor that when flying in cool weather and there is any precipitation.  You want at least a 5 deg C margin above freezing at an absolute minimum, and if inexperienced probably more like 10.  When I was bushing flying I would do trips in late fall where it was raining and the freezing level was only a thousand feet AGL or so and I had to stay at 500 ft off the treetops  because if I went much higher I'd start picking up ice.  You do some crazy stuff while in the bush.


text:
On ATC's radar, does an aircraft that loses its transponder simply disappear?
If you look at shows like Air Crash Investigation, which surely take some creative license in how events are portrayed, they often portray the disappearance of an aircraft, or specifically loss of its transponder return, as the radar return simply disappearing from ATC's radar scopes.
My question is: is that actually the only thing that happens when ATC secondary radar loses an aircraft's transponder return, when the aircraft is not actually on approach to land (and thus losing line of sight to the radar would be normal and expected, and/or that radar return is no longer relevant for air traffic control purposes)?
In general, humans are better at picking out things that are there than they are at picking out things that aren't there, even if they were there previously. It's also very rare for an aircraft to lose its transponder, let alone crash. Therefore, some kind of positive notification that a transponder return was lost when it shouldn't have been, possibly after a short delay (to account for those aircraft where the transponder is set to standby while a new code is being selected) would seem helpful.
Yes, it's the controller's job to keep track of the aircraft they are in charge of, but especially with modern, digital radar systems, it seems like something like flashing a distinct symbol where the aircraft was last seen, or sounding an alert chime, or just about anything, could make their job a lot easier the very few times that an aircraft transponder return disappears when it shouldn't have.

In the US, when on flight following while practicing approaches, I had the transponder go out because of water in the antenna. ATC lost contact with us and had us do a couple of turns to headings so that they could identify us on primary radar. They were able to vector us for approaches, so their system had some way of keeping track of us even though they couldn’t pick up our transponder.

At least some ATC radars are perfectly capable of picking up objects that don't have transponders.  I've had them notify me about targets that turned out to be hang gliders, or even flocks of birds.

It depends on the system. Assuming only secondary radar is available, in some systems, the track will indeed just disappear when radar contact is lost. Other systems will put the track in "coasted" mode, continuing to show the track (but with another track symbol), waiting to pick up the transponder again. The track will then be terminated after a specified time limit.
Of course, if primary radar coverage is available - which works independently of aircraft equipment, such as transponders - the system would still be able to track the aircraft, but automatic flightplan correlation would most probably be lost, along with mode C data (altitude). So all flight information would vanish from the screen and the controller would have to re-identify the flight using proper procedures.
Producing an alert when a track is lost could be useful, but it could also potentially result in a lot of nuisance alerts, particularly in areas with spotty radar coverage. So alerts should only be shown for areas and levels where radar coverage is expected to be perfect, otherwise the controllers would likely develop a habit of ignoring such alerts. Also remember that radars lose contact with flights all the time - as they move out of the range of the radar. No ATC station has global radar coverage. Showing an alert every time an aircraft leaves the area of radar coverage would also be pointless, and the range of a radar can vary from day to day depending on atmospheric conditions, so defining a precise area in which alerts should be suppressed might not be a trivial task.

Nice answer! Only thing to add is, that most of the airspace is covered by primary radar as well. In that case the radar label world disappear and so does all the Information (squawk, level, callsign, mach number, heading,...) but the target with Speed and speedvector would Be tracked normally

"otherwise the controllers would likely develop a habit of ignoring such alerts" I'm sure that, if the desire to do so is there, there would be a way to unobtrusively indicate to the controller that the transponder return has been lost. Even just changing the symbol to add a question mark (or whatever else is unused and clear) could unobtrusively draw attention to a specific radar return. As for when aircraft leave the radar coverage area, that sounds like a place where adaptive thresholds could be used to good effect. Neither seems an insurmountable problem in the least.

@aCVn what happens in a primary capable facilitiy is that the symbol will change to denote the loss of the secondary radar return, AIUI

@aCVn Not saying the problems can't be solved. Just explaining how it works.

@pcfreakxx, if the track had a flight plan correlated with it, some information (callsign, cleared level, etc.) won't be lost. And besides, most of the airspace is certainly not primary radar-covered. All oceans, for example; most of Siberia and Australia...

@Zeus The system behaviour depends on how the tracker is designed. I know two systems- 1 would continue correlating the track while throwing a warning and the other one would just drop the correlation. I was Talking about central europe where we do have a good primary radar coverage. Really important to see the wingmans of all the F35 and F16 formations flying around!


text:
Does flying an airplane upside down imply negative AoA? Why?
I saw the statement below in this answer:

Note that for normal level unaccelerated flight $C_L$ is always > 0. The minimum $C_L$ is a negative value for negative AoA, and would mean that the aeroplane is flying upside down.

This would seem to me to imply that flying an airplane upside down implies a negative angle of attack.
But the typical illustration and explanation of angle of attack is that it is the angle between the airstream and a reference line, the latter of which in airplanes is normally the wing chord.
How can the two be reconciled? Alternatively, what am I missing?

I don't understand why the two statements need reconciling.

Negative AOA to generate negative lift. So flip it upside down and you have positive AOA...

Level flight (no change in altitude) upside down, with an airfoil with a zero-lift AoA of less than or equal to zero degrees, requires a negative AoA to get upwards lift.
This is assuming the AoA is measured in the body frame of the aircraft, aligned with the chord line of the airfoil.
Also this assumes there is no thrust/drag or thrust/drag do not meaningfully contribute to vertical net forces.

It depends on what you are using for your coordinate system.  If you are using a coordinate system relative to the airfoil where the top of the airfoil points towards the postive y direction and the bottom of the airfoil points to the -y direction, then inverting the airplane will cause a negative AOA since the airfoil would be fixed in this coordinate system.  In this case the negative AOA would cause negative lift as defined in this coordinate system.  However, if I defined the coordinate system relative to the ground where -y points towards the ground and postive y points towards the sky, then the AOA would remain the same in both normal and inverted flight and the AOA would be positive with positive lift in both cases.

We could pick the system of co-ordinates where AoA does not depend on which side of the plane is the "upper side": the side facing the sky could be the upper side, regardless if that side has landing gear on it or not.
This would look more intuitive for human to understand. However various algorithms of computer simulation would have difficulties at the 90 degrees roll. At this angle the AoA would reverse the sign, because the "upper side" is now the opposite. Such changes make tasks like calculating derivatives over this point awkward.
The math is much easier without the badly-behaving points. This is generic everywhere in mathematical modelling.

"The minimum Clift is a negative AOA value"  means that positive lift is still being generated at a negative angle of attack.  Go to Airfoil Tools on the net and study the graphs.  Many cambered airfoils do this down to a few degrees negative AOA.
For aerobatic planes, symmetrical airfoils are used to get similar lifting properties when trying for sustained inverted flight.  They will generally will have 0 lift at 0 angle of attack.
So "negative" or "positive" lift really depends on which direction you want the WING to generate lifting force.
For example, if you enter a loop, you may be inverted but pulling back on your stick to complete the loop.  AOA is still positive.  However, if you do a half loop and wish to fly straight away inverted, you now push your stick forward, to negative AOA, creating lifting force towards the bottom of your wing (which is now "up").
When you half roll back to upright, then pull the stick back for positive lift.
This is the "Immelmann" maneuver, from early in the last century.


text:
What are the dynamic stability properties of an aircraft with wings exactly at the vertical CoG?
Please ignore for a moment the practical issues with building and operating an aircraft like this.
Suppose a fixed-wing aircraft which, when viewed directly along the roll axis, has the wings centered exactly at the vertical center of gravity. Suppose that this aircraft is in stable, level flight in otherwise still air. (Yes, this is a little like the spherical cow in a vacuum from physics.)
If there is a disturbance, such as a localized updraft or downdraft affecting one of the wings more than the other, then broadly speaking and as I understand it so far (feel free to address this if I've got it completely backwards), after exiting the disturbance and with no further control inputs, a high-winged aircraft will tend to return to neutral roll (dynamically stable) and a low-winged aircraft will tend to increase its roll angle further (dynamically unstable), as a result of the differences in localized airflow above and below the respective wings.
However, what would be the behavior in such a situation of our hypothetical mid-winged aircraft that I described above?
My hypothesis is that the aircraft would maintain its roll or bank angle, which (unless compensated for, say by rudder input) in turn will result in a rotation along the yaw axis and a subsequent turn.
Am I right or wrong? Why?

If someone knows of better tags, by all means feel free to retag.

It is a common misconception that the vertical CG position (relative to the wing) plays a major role in roll stability. It doesn't; it's mostly about aerodynamics. (You mention it yourself in "as a result of the differences in localized airflow...")
There are other important factors at play as well: dihedral angle, wing sweep, design of the vertical stabiliser.
Presumably, you are interested in the behaviour of an aircraft with the perfectly neutral roll stability. If so, it needs at least these conditions:

General symmetry about the horizontal plane, which includes:

Zero wing dihedral;
Symmetric (e.g. round) fuselage;
Wing at the centre of fuselage (vertically);
Symmetric vertical stabiliser (e.g. with dorsal fin);
Absolute rigidity so that the symmetry remains under load.

Zero wing sweep (straight wing);
Lack of any artificial stability augmentation.

(Technically speaking, the geometric symmetry is not necessarily required: we can compensate one effect with another, say, wing sweep with anhedral. But in the spirit of the original question, let's assume the full symmetry).
We should also assume a reasonable stability in pure yaw.
So, if you induce a pure roll disturbance and then remove it,

the aircraft will start rolling.

Because it has some inertia about the roll axis, it will not simply stop when you remove the disturbance. The only force that will counteract the roll now is roll damping. It naturally occurs because the descending wing has higher angle of attack (and vice versa). The strength of the effect depends on the square of the wingspan, but in theory the spherical cow will keep turning forever, albeit with ever-diminishing speed.
So far, nothing would induce a yaw, and the airflow would still be symmetric.
However, as roll develops, and the aircraft keeps the same pitch balance, the lift will become insufficient to keep it level. The usual thing happens:

the aircraft enters a spiral dive.

Now, this is not a symmetric situation anymore. There is a side force from gravity; sideslip will develop; the aircraft will yaw due to its yaw stability that we assumed. Things start to get complicated (that is, 'normal'), with many variables involved. Even in the vertical plane, despite postulated symmetry, the shadowing and other cross-flow effects on the top and bottom of the wing will be different (the wing, by definition, cannot be aerodynamically symmetric when it generates lift), and thus some roll effect may be observed.
So it's hard to predict further behaviour in general. At one extreme, with stubby wings and slender body and very strong initial disturbance, the aircraft may barrel roll many times and may even enter a peculiar stable roll motion due to inertia coupling. At the other, the aircraft will just roll a little and will enter a gentle (nearly) coordinated turn with very slight descent.

In order for an aircraft to return to neutral roll the shape of the wings is an important factor as well and a number of other aerodynamic factors will play a role. A positive V-shape for example always helps to return to neutral. Furthermore wings are sometimes also designed to bend under load too which will have the same effect as the V-shape.
But if cows are spherical I would say your hypothesis is right. On high-winged aircraft the leverage of the lower wing when the aircraft is banked is longer than that of the upper wing resulting in a torque in the opposite direction of bank; resulting in a decrease of bank angle. If the wing is placed on the center of gravity the leverages of the forces on both wings would be equal regardless of bank angle and thus the total torque would be zero; the bank angle would stay as it is.

There is no leverage either way. The centre of gravity is in the action line of lift whether it is above or below the wing, so it's not creating any moment at all. Only lateral aerodynamic forces due to side-slip are creating rolling moments.

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A scenario question regarding meteorology
In the image attached, I’m struggling to find out why the answer is the way it is; maybe is me not being able to visualise it, but some help would be greatly appreciated.


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What exactly happens to kinetic energy in the turbulent layer on an aerofoil?
Sources I've read contains contradicting information.
As far as I know, when the laminar airflow breaks up, it becomes a thicker and draggy layer, causing a layer of what essentially is slowed air; this would suggest a reduction in kinetic energy, however some books have stated that it is actually an increase.....?
This makes what seems to be a simple concept to be hard to grasp, personally I see it as a reduction in KE, but the books say otherwise. So if anyone knows what happens to KE in a turbulent layer, that'd be greatly appreciated.

Energy is transferred from the moving airplane's skin into the air in the turbulent boundary layer. Because the layer is turbulent, all the momentum vectors for every little parcel of air entrained in the layer are pointing in random directions with some uniform component (in the direction of the plane's travel) superimposed.
Thus the air in that layer gets vigorously stirred up, and since air has viscosity, the stirring action gets fairly quickly transformed into frictional heat. So after the plane has departed out of the picture, the turbulence and net motion in its wake get dissipated and the air is left a little warmer than it was before the plane flew through it.

In the reference frame of the aircraft, no energy can be transferred from it (work is force times distance and the wing isn't moving). In that all the extra energy is at the expense of the pressure energy, which is why turbulent layer increases lift and delays separation (stall). Of course pressure is independent of reference frames.


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What is the difference between AHRS and ADIRS?
Whats is the difference between Attitude and Heading Reference System and Air Data Inertial Reference System ?

The navigational avionics systems have developed to a point where multiple systems currently co-exist with their design and introduction dates ranging from the last decade to the beginning of the 20th century, owing to the wide range of aircraft out there.
Specific to your question, this led to some quite confusing nomenclature, due to the complexity and integration capabilities of the technology.
The key is to understand that a NAV system comprises of a number of subsystems, each then having several components.
AHRS and ADIRS, although each have the term “system”, are actually on two different levels of the NAV systems’ structure (or, hierarchy), with AHRS being a sub-system (from the perspective of ADIRS).
ADIRS: Air Data Inertial Reference System
The designation for a total NAV system comprised structured as follows:
ADIRUs - Air Data Inertial Reference Units (which are then comprised of an Air Data Reference [ADR] and an Inertial Reference [IR, or IRU] components
Probes - the Pitot tubes, static pressure ports (often combined in one Pitot/Static probe) and Angle of Attack vanes
ADMs - Air Data Modules, units which compute the physical data of the probes into numerical information; and then feed that into the Air Data Reference unit of the ADIRU for processing
CDUs - Control/Display Units, which use the data processed by the Air Data Reference unit, to display it on the EFIS (or present it to the pilot on the CDU’s display)
AHRS: Attitude/Heading Reference System
The designation for the system (which can be part of the larger structure of the aircraft’s NAV system, or it may be the only system providing navigation) comprising of two components:

sensors for the three-axes of the aircraft’s pitch, roll and yaw and a sensor for the Earth’s magnetic field;

-an integrated computing unit which processes the sensed information into - Attitude and Heading, respectively.
Some additional points regarding AHRS:

this system can use either mechanical sensors for rotation (gyroscopes), motion (accelerometers) and magnetism (magnetometers), or it may use the more recent MicroElectromechanical sensors (MEMS) technology.

AHRS may then be also combined with an Air Data system (an Air Data Reference Unit) with the two then forming the Air Data + Attitude/Heading Reference system (ADAHRS).

the AHRS may be part of the Inertial Navigation System (INS), where the AHRS (or ADAHRS) in the INS is what the IRU (or ADIRU) is to the ADIRS.

the difference between AHRS and the Inertial Reference Unit /
IRU / Inertial Measurement Unit IMU [two interchangeably used terms] is that the IRU/IMU does not have an integrated computational part for processing the measured information but rather just feeds data to another unit of the NAV system’s structure. Actually, the sensor component of AHRS is, in fact, its IRU/IMU.

Note:
The IRU/IMU may use any of the technologies used for measurement (Gimbal-suspended/Fluid-suspended Gyrostabilized Platforms, StrapdownSensor systems, etc.). However, many aircraft use the modern RingLaserGyro technology. Multiple RingLaserGyro-based IRUs are often termed as an “Inertial Reference System (IRS)”, and this adds confusion - because, an INS using any base technology or system structure (i.e. an INS using an AHRS subsystem) are also sometimes called “IRS”, and the “IRS” in ADIRS does not imply RingLaserGyro technology must be the base for the IRUs.
A conclusion is to understand that an aircraft navigational system varies in complexity and structure which may lead to some Tetris-style nomenclature:
from smaller GA airplanes having independent instruments (Mag compass and attitude indicator), across the older turboprop fleet/smaller jets having more systematized and inter-connected structures (a single air data computer ADC/ADR with an AHRS) towards the newer aircraft and larger turbos/jets with two or more ADCs/ADRs, IMUs/IRUs and both subsystems being part of a larger system.

The most important difference is that an ADIRS contains an inertial system, which gives a position estimate and can be used for navigation. This is done by aligning to a known position on the ground and then integrating all changes in orientation (measured by ring laser gyroscopes) and velocity (measured by quartz accelerometers) to update the position. As a by-product you also get the orientation (attitude and heading).
The AHRS only gives this orientation information, but no position. It therefore replaces conventional gyroscopic instruments like a directional gyro and an artificial horizon. An ADIRS can usually be switched to ATT mode (instead of full NAV mode) in flight and then function as an AHRS without navigation capabilities:

(source: 737 NG FCOMv2 (11.10.21 - Flight Management, Navigation - Controls and Indicators))
The ADIRS also performs all computations done by an Air Data Computer (ADC), which usually comes installed separately next to an AHRS for glass cockpits, and therefore replaces the conventional pitot-static instruments.


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How does the FAA decide if a PPL applicant with ADHD can fly?
For the past few years, I've been struggling with fidgeting, difficulty spending long periods of time on creative tasks, difficulty finishing long and complicated tasks, and a few other symptoms. These are difficult in my professional life as I'm a journalist, so long periods of time doing research and writing is a necessary part of the process. My physician mentioned ADD/ADHD as something to look into, with potential benefit if treated with medication.
The trouble is, I'm also looking into getting my PPL next year, and I noticed that the FAA outright bans a number of ADHD medications, and seems to have a whole extra evaluation process you have to go through if you get diagnosed with ADHD. Nowhere could I find a good explanation of what this means if you're considering getting ADHD treatment but you also love, love, love to fly.
What is the FAA's policy, specifically covering PPL medicals for those being treated for ADHD?

Welcome to aviation.SE! This question comes up regularly on aviation discussion forums and you might like to check some out (if you haven't already) to learn about other people's experiences. Unfortunately an ADHD diagnosis seems to make it long, difficult and expensive to get a medical but everyone is different and only an AME can help you with your specific situation. You can also look into flying as a sport pilot. You don't need a medical for that, but if you've ever failed an FAA medical then you can't fly sport either so it would be best to research thoroughly before making any decisions.

@Fattie Agreed! I would mention that typical TV depictions of ADHD are far more severe than most real-world cases. Generally, ADHD looks less like assault and more like trouble focusing for long periods of time, jumping from topic to topic in conversation, and restlessness. As far as focusing it's really creative tasks like writing that get me, and not a problem when flying in my experience, and I can drive for far hours and hours on end without getting bored or distractible. Obviously though, public safety must come first, which has unfortunate consequences for those of us don't threaten it.

@Pondlife Thanks. I've been reading up on the forums, and what really boggles me is the inconsistency. Countless varied experiences across the community, with equally variable outcomes. I'm trying to find a single solid description of policy as it applies in actual cases with actual outcomes, and I'm worried about talking to an AME in case I say something that might come to haunt me. For example, "trouble focusing" can easily be interpreted as "trouble paying attention to flying the plane", even if this description is far from accurate in my case

@Fattie, in the United States there is a right to fly an airplane.  It isn't spelled out specifically, but then neither are most of our basic freedoms.  It is covered under the umbrella right to "life, liberty, and the pursuit of happiness."  Just because you have to qualify medically, demonstrate proficiency to operate safely, and are required to follow rules, doesn't mean you don't have a right to fly once you've earned your wings.

its splitting hairs.  I have a right to drive a car; I don't have a right to drive a car under many conditions.  No big deal.

This article from AOPA outlines the issues with ADHD pretty well.
This article is from Canada but the medical policies are quite similar and you might find the basic information useful.
Bottom line seems to be that once you are formally diagnosed with the condition, that becomes a problem.  You can't be on ADHD medications at all (within the last 6 months) and if you are diagnosed (officially) and are not on medications, you have to go through a testing protocol to satisfy the FAA that are fit to fly without medication.
There seems to be a lot of misdiagnosis of ADHD and the symptoms you describe may not be severe enough to be considered a real case (you may be getting too hard on yourself; I get sidetracked on tasks by things like shiny bottle caps on the floor, get bored easily, and if you give me a Rubik's Cube, I'll tire of it and put it down after about 2 minutes, and I've been flying since the mid 70s).
When I read about the symptoms of hard core ADHD (especially over-diagnosis of kids - it's criminal in my mind) it sounds like you would be barely able to function in the world, and your description doesn't sound like it rises to that level.  I'd be wary of proceeding along the path to an official diagnosis unless you've decided it's so bad you can't function without medication, and at that point flying is out.
If you do nothing and carry on, and you have a significant case, it'll become obvious during training and you'll probably never get to the solo stage.  FAA protocols are based on a diagnosis, which should be by a psychiatrist, so I wouldn't bring it up to an AME until you get to that point.  I would explore other non-psychiatric therapies or techniques that could help with focus and discipline first, and would avoid the medical route unless it was a last resort.  Your doctor may not have been doing you any favors by suggesting you might have a serious psychiatric condition based on some comments.

The only benefit to being officially diagnosed is that the doctor can prescribe meds, and since pilots are not allowed to take them, all you’re left with is the cost.

It's a matter of human performance limits -- which is a required part of PPL training in Germany at least -- and pilots are expected to make an honest assessment of their ability to fly. Doctors can and should assist in this. Whatever rules exist are there because people in the past failed to do this, and should not be taken as advice on what is barely permissible, but what certainly is not. Hyperfocus can be a good thing, but turns bad if you miss radio transmissions because of that.

That's not a formal requirement in North America but someone with real ADHD will be extremely difficult to train, and won't be able to hide it, and it's extremely unlikely they will get far at all.  If someone is in the US in that situation, then if they want to fly they could just stick with Part 103 ultralights or paramotors where no lic or medical comes into it.

Real-life ADHD isn't like in the movies; most cases are far less severe, and I'm sure you know at least a few people in your life with ADHD who haven't mentioned it. While there are definitely plenty of people with ADHD who I wouldn't want flying a plane, I know far more people whose symptoms are minor and limited in scope to particular situations (research and writing, for example), and could complete training with little to no difficulty beyond that experienced by anyone else. The concern is mandatory disclosure, which doesn't seem to include a checkbox for "some conditions apply"

When something is on a spectrum like that, you end up capturing a pretty big segment of the population and the definition can become effectively meaningless.  Applying that kind of psychiatric label to people who simply get bored easily with certain tasks, and which sometimes can be simply a side effect of very high IQ, is pretty dangerous in my view, but I have an old-timey outlook. The prospect of that label closing doors certainly concentrates the mind.  Mandatory disclosure only applies to a formal diagnosis so I wouldn't even go there unless you've decided you need prescribed medication.

I gave an intro flight to a man with ADHD who wanted to become a pilot. He loved it so much we started lessons.
On lesson #1 he realized that he was unable to focus enough to complete the after start checklist. So he and I decided to call it quits right after engine start. Actually, he realized it wasn't going to work before I had said anything.
This man did not have uncontrollable fidgeting. If he had, we wouldn't have begun lessons.
Piloting requires steady hands, focus, follow-through, and remaining calm in all situations.
If I were you I would go to a flight school and buy one of those $120 "intro flights" to give yourself an opportunity to see how your mind acts while flying an airplane.  You may not get to fly too much, other than some straight and level and a turn on a short intro flight, so you might need to do an actual lesson after that one, but maybe you'll get a chance to see how it makes you feel.
Speaking of my personal opinion, I'm not a fan of going and getting a diagnoses that makes you define yourself as having some limitation.
This isn't about rules outside of your control limiting you from things you want to do, this is about you knowing and understanding yourself and knowing what you are or are not capable of doing.
"Know thyself".
Go do a couple lessons and see how YOU react. Tell your instructor that you think you have ADHD (even to include fidgeting), and see if he/she is willing to take you up (there are two sets of controls).
Regarding the fidgeting, this likely does not apply to your situation, but if you drink diet soda/pop, stop.  Aspartame toxicity presents to doctors in a way that they diagnose as Parkinsons.
Avoid stimulants like caffeine (especially energy drinks), drink lots of water.

This. Flying at any "level" requires ability to focus and perform tasks meticulously. Any difficulty with those is an invitation to dance with death.

Great advice, no sense worrying about the admin until you know you are capable first.


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Is it possible to stall a stationary helicopter through excessive collective input?
I understand that helicopters can suffer from "retreating blade stall" where the retreating side of the rotor has low airspeed and stalls in a typical way that airplane wings stall. This happens when the helicopter is underway.
By contrast I am interested if it's possible to stall in a hover or at very low speed, with normal rotor rotation speed, by pulling up too hard on the collective controls, generating an excessive angle of attack for the blades. Is it impossible because of the lever effort that would be required? Are there any safety systems that prevent it? Is it simply impossible to dial in enough collective to cause the blades to stall, even with the lever pulled all the way?
If it is possible, how do pilots know how much collective input is excessive and could cause a stall?

Helicopters use speed of the wing as a proxy for angle of attack in a similar manner to airplanes; in the airplane's case it's forward airspeed and in the helicopter's case it's "rotary airspeed", expressed as rotor rpm.  In the airplane you have the bottom of the green arc on the airspeed indicator; in the helicopter it's the bottom of the green arc on the rotor tach.  In the airplane there's a stall warning horn; in the helicopter there's a low rpm warning horn (that's the only "safety system" to prevent blade stall).
If you're in a hover, there's nothing stopping you from yanking the collective pitch all the way to the up stop.  This shouldn't be sufficient to induce blade stall because the machine will respond by both climbing and increasing the vertical downwash of air, both of which will make the actual blade AOA increase less than the physical pitch change made by pulling collective, as long as rotor rpm stays in the green arc.  Whether this action keeps the rpm in the green is another matter, and on turbine machines it's likely to overtorque/overtemp the engine(s) even if rpm is kept in the green.
If you pull pitch like that and the engine can't deal with the load even at maximum power, the rpm will quickly decay and once it goes below the green arc, blade stall could (will) occur at some margin below the green arc.  When blade stall occurs from low rotor rpm while in a hover or at very low speed, the rpm decays even faster, the blade drag is too high for the engine to fight against, and the machine starts to settle vertically, causing blade AOA to increase further.
Before you know it the blades are WAY above stalling AOA, and you become While-E-Coyote strapped to an anvil and down you go; there's no recovery once a vertical descent with a stalled rotor starts, engine or not (if this happens in an engine out autorotation at altitude, the rotor will be close to stationary by the time you hit the ground - I recall a Robinson R22 incident from the 90s where the pilot lost engine power due to suspected carb ice, while flying along at 1000 ft, and didn't lower collective in time to get a proper autorotation started, the rotor rpm quickly decayed, the rotor stalled, and the machine just fell out of the sky like a wounded goose).
So the limitations are not so much about collective movement, it's the effect of collective movement on rotor rpm (and engine limits) that is critical, and the pilot has to monitor rotor rpm and respect the bottom of the green arc and the rotor rpm warning horn to stay safe, knowing that if the limits are exceeded and a stall happens, there is probably no recovery.
So, if you are hovering 10 feet in the air and you let it happen, it's a hard landing with maybe the tail boom getting chopped off.  If it happens while hovering at, say, 100 ft, the result is fatal.
To answer each question specifically:

Yes you can pull full collective any time. But the key safety parameter isn't collective position, but rotor rpm.
The only safety system is the rotor tachometer and the low rpm warning horn.
Yes, IF the engine has sufficient power to maintain rotor rpm in the green while pulling full collective. Some helicopters may be able to do this some may not.  If it can't, the rotor stall won't occur until the rpm decays to some point below the green arc, and once that happens, you are done, even if the engine is still making power.
Excessive collective input is revealed not by the actual lever movement, but by its effect on rotor rpm, which is the most critical parameter for the pilot to monitor. See here for a Robinson safety letter that talks about low rotor rpm rotor stall to see just how critical this parameter is.


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Can there be interference drag in inviscid flow?
I understand that interference drag is the difference between the terms: the total drag of a configuration, and the sum of the individual drags of the components in the configuration.  (It exists due to flow field interactions)
So if we assume a non lift generating configuration and components at subsonic speeds (so no induced or wave drag), is it right to say that there will be no drag in an inviscid flow as both the terms would reduce to zero (no form or skin friction drag) resulting in zero interference drag?

It is not always easy to know that your lift-free objects (when separate) remain lift-free when brought into close proximity.  The total lift can be zero, but if there are forces anywhere within system, you will have induced drag.

If you assume the flow to be incompressible, inviscid and stationary then the Navier-Stokes equations reduce to the Laplace's equation and the relevant aerodynamic force posseses no drag if the body is either 2D or 3D but not generating lift anywhere - just like in your hypothesis.

A non-lifting body in inviscid flow can have induced drag if there is lift (that cancels) on parts of the body.
Imagine two rectangular, untwisted wings with symmetrical airfoils -- arranged in a biplane configuration.  They are connected together with end plates simply to make them a single body.
If the chord lines are parallel and the configuration is at zero alpha, it will have zero lift (and should have zero drag).
However, imagine the chord lines have +-5 deg incidence.  Each wing will experience an equal and opposite lifting force -- each generating induced drag.  However, the total lift will be zero and the total drag will be non-zero.
You can accomplish the same thing with a twisted wing -- twisted (even cambered) wings will have non-zero induced drag at the zero lift condition.
You might not consider this interference drag.  I would probably agree with you.
To me, the distinction is that if you move the two wings arbitrarily far apart -- but still consider them a single body -- you get the same effect.  This is simply two wings with equal and opposite lift that result in zero lift and finite induced drag.
However, let us go back to the zero-incidence biplane.  If the two wings are 'close' together, then there will actually be a 'venturi' effect (due to the airfoil thickness) between the airfoils.  This will cause the flow acceleration 'between' the wings to be greater than on the 'outside' of the wings.  This will cause (cancelling) lift forces -- that will have associated induced drag.

In this analysis, two NACA 0012 wings were placed a half-chord apart.  Zero incidence, zero angle of attack, etc.
You can see from the shape of the pressure coefficient contour lines that the flow on the top of the top wing is different from the flow on the top of the bottom wing.  This is because of the super-acceleration in-between the wings.  In this case, the bottom wing will have an 'up' lift force and the top wing will have a 'down' lift force.

Here is the lift distribution on these wings -- the important part is that they are non-zero and that they are equal and opposite.
Because they are non-zero, they will each have induced drag.  However, the total lift will be zero.
I would say this is properly an interference drag.  If you move the wings far apart, this effect will go to zero.  If you move them closer together, this effect will increase.
That said, this effect is really small.  However, it is exactly the root cause of viscous interference drag.  If you place an object in a flow with other bodies such that it experiences super-velocities compared to when it was in isolation, the viscous drag will increase just as if it was in a faster flow.

Great explanation!


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Why there are no fully articulated 2 rotoblades helicopter
Why can't R22 and R44 make the 2 rotoblades fully articulated
Like cabri G2 helicopter but only 2 blades
So that easier to store in the hanger than 3 blades rotor blades helicopter and no mast bumping problem like semi rigid R44 mast bumping problem

I do not know for sure but my suspicion is this…  You get a lot of simplicity out of a rigid rotor, and a lower cost to build, maintain, and replace.  I think that once your design criteria moves up into the domain of fully articulated hubs, then you are also in a design and mission space that benefits from multiple blades.  So I think that it’s not “two-blade tutors have rigid hubs” but rather, “articulated hubs tend to have more blades.”


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Does anyone know what specific plane this is a model of?
Curious about the one 6 seconds in on this compilation of flutter experiments

Considering it failed, I'm fairly sure it isn't a model of a existing (at that time) real airliner.

My guess is that it is a B-52.
The YouTube description also says that one of the aircraft is a B-52.

Kev Cook B-52 Flickr photo

When I saw the tail, the B-52 immediately came to mind.

Wikipedia XB-52 photo

I thought about this as well, but it seems odd that all 8 engines are missing. I think the weight and aerodynamic influence of engines is rather important for aero-structural experiments

Also the plane in the video has considerably wider wing root, and a different fuselage cross section and overall form.

the thin swept wings on the B52 were very controversial and a cause of concern. This was most likely a test of an early design. The amount of flex in a B52 wing is almost unbelievable. It caused many issues over the years. The wing tans were originally drop tanks, but dropping them would have caused the wing tips to flutter. Tip gear had to be added to prevent the wings from hitting the ground. The B52 also had wet wings, the skin of the wings were part of the fuel tanks. Given the amount of flex we always had fuel leaks. Especially we add the weight of the ALCM pylons under the wings.

It looks B-52ish but may be some sort of generic design fit scientific study in a wind tunnel than aerodynamic research for an actual production aircraft.


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Do wingtip vortices really only form when airplane wheels are separated from the ground?
My GA theory textbook (which covers fixed wing up to and including EASA PPL) has some suggestions on taking off and landing light aircraft soon after large aircraft (besides avoiding it, if possible), intended to reduce the risk from wingtip vortices. These suggestions are to,

when taking off

...after a heavy aircraft taking off, to lift off at a point on the runway before the heavier aircraft lifted off
...after a heavy aircraft landing, to lift off at a point on the runway later than where the heavier aircraft's nose wheel touched the runway

when landing

...after a heavy aircraft taking off, to land as early as possible on the runway
...after a heavy aircraft landing, to land ahead of the point on the runway where the large aircraft touched down

When I asked about the why of these suggestions, the answer was that wingtip vortices form because the wings are generating lift, and when the aircraft's wheels are on the ground, the wings don't generate (or aren't generating) lift so no wingtip vortices form.
While that might be true in a "rule of thumb" sense, and a good generalization of these rules to serve as a memory aid, I want a bit more than that. The way I understand it,

Any airfoil will generate some amount of lift as long as there is ambient air movement around it; "lift" (as opposed to net positive vertical lift, which isn't being generated when landing anyway because the aircraft is descending) isn't something that suddenly starts being generated when one or more of the aircraft's wheels are separated from the ground
An aircraft can move at significant speed relative to the surrounding air even when all of its wheels are on the ground, causing likewise significant speed of movement of the wings through the ambient air

It appears to me that wingtip vortices should form at any time the aircraft (or rather its wings) is moving relative to the surrounding air; whether that's because the aircraft is standing still and there's a wind, or because there's no ambient air movement and the aircraft is moving, or a combination of the two. It also appears to me that the strength of the vortices should be in some way proportional to the amount of lift being generated.
Consquently, it seems to me that wingtip vortices should form at every point during the takeoff or landing roll (as well as taxiing and other movement on the ground) at which the aircraft is moving relative to the surrounding air, and increase in strength with (but not necessarily linearly with) the indicated airspeed of the aircraft.
It seems weird that the air flowing near the wingtips would somehow "know" anything about the position of the aircraft's wheels with respect to the ground, let alone that of any specific wheel (such as the nose wheel).
All that just as the lead-in to my question: am I missing something, or is this a case of the textbook (and teacher) presenting the simple case so as to not overwhelm students?

You are absolutely right. The vortices form whenever the wings produce lift. Even if the lift is not enough to let the aircraft fly.
Of course it is necessary to have a flow across the wing, and with very low speeds (like taxiing) the vortex is likely to be very small.
But,
it seems your textbook is trying to establish a save way to avoid the vortices of previous aircrafts. And since the strength of the vortex is related to the lift which is related to the air-speed and angle-of-attack this rule of thumb is quite practical. Especially since landing is a highly transient process which is very likely to result in a significant reduction of vortex strength.

Very close to the ground wing tip vortices are also less important because they dissipate much faster due to the ground effect. The ground basically 'blocks' them.

You are correct, but your textbook is more correct. Here's why:
Your reasoning is that lift is a function of airspeed, which is true. However lift is also a function of angle of attack. This is why large aircrafts take off the way they do: they accelerate on the ground with a very small AoA, hence little lift. Since lift increases with drag, by accelerating when producing minimum lift, drag is reduced, and the acceleration to takeoff speed can be achieved on a shorter runway length. By pitching up at the right moment, lift is dramatically increased, and lift off is accomplished.
The reverse is true when landing. The purpose of the landing rollout is to absorb the kinetic energy of the plane in the brakes. Having the wings create lift reduces the friction between the tires and the ground, which does no good at all. Therefore lift is dumped as soon as practical during the landing rollout.
It should be noted that, one technique to achieve a short field takeoff in a light aircraft is to accelerate with no flaps. Then, when the takeoff speed is reached, quickly apply a notch of flaps and the plane will lift off the ground. This technique is usually not taught to new pilots because it requires skillful coordination, but it illustrates that lift is suddenly increased at the point of lift off.
Another way to phrase it would be: when the wheels are on the ground, the amount of lift generated is small and usually within the tolerances of a light aircraft. But you are correct in saying that lift is still generated as long as there is relative movement between the wing and air.

When the aircraft is on the ground, the wheels are supporting it. When it is in the air, the wings are generating the upward force necessary for lifting it. So at take-off, there is a sudden transition from wheels to wings in the lifting function.

Any airfoil will generate some amount of lift as long as there is ambient air movement around it;

That depends on the angle of attack:

symmetrical airfoils generate zero lift at zero AoA;
a-symmetrical airfoils generate zero lift at a slightly negative AoA;
airliner wings have wing twist: the area near the tip can have negative AoA when on ground;
the overall AoA of aircraft wings with short nose gear is negative when on ground.

Iron birds at an aircraft factory that go through their fatigue cycles simulate the starting cycle by actuating the wing up/down during taxiing (from bumping on the taxiway), then during the takeoff run the bumping intensifies, then at the take-off nose-up motion the wings bend upwards dramatically - one can hear them groan and creak while they bend.

"lift" (as opposed to net positive vertical lift, which isn't being generated when landing anyway because the aircraft is descending) isn't something that suddenly starts being generated when one or more of the aircraft's wheels are separated from the ground.

Strictly speaking there will be some downward force generated by the wings, but the transition from wheels to wings is so dramatic that for practical purposes we can indeed state that wingtip vortices are only significant with the wheels off of the ground.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
If a jetliner is giving TOGA thrust but never releases the (parking) brake, will the brakes heat up?
From my - admittedly - limited knowledge of physics, the energy generated from the engines needs to go somewhere; will it go into the brakes or does it just dissipate into the air?

Aren't the real questions: Why would one do so, and how long will it take to realize the mistake (if not indented)?

@U.Windl it was more a hypothetical scenario anyways. I know there are plenty of safety features that this shouldn't happen (for too long)

Also a quick "thank you" to everyone who commented here, I appreciate it :)

This question deserves a lot more upvotes, simply because of the amount of debate it sparked. Well done Nico!!!

Absolutely, and now I ask: name one form of propulsion that does anything other than to impart force on what it is trying to move?  Simple thought experiment.  Take a block of wood and set it on the table.  Apply force with your hand.  At  will accelerate until force = friction (and aerodynamic drag!).  The  Power of the block is Force x Velocity.  Now nail the block to the side of a merry go round. Relative to where you place it there is no V.  But the block + the merry go round recieve the force, same as the jet and earth.  Easier to see if the merry go round moves.

Brakes absorb energy through dynamic friction. If the plane doesn't move, the brakes do not absorb any energy, and do not heat up.
The energy is going into the air either way, whether the brakes are applied or not.
The only difference a parking brake makes is whether the reaction force, arising from Newton's third law, acts on the aircraft alone or on the aircraft+Earth system. Momentum will be imparted to that system.
The additional ballast of M⊕ is about 1019 times the weight of the heaviest airplane, so it reduces its acceleration by a factor of 1019. Planets are a heavy load to move.
More technical:
The amount of momentum imparted to the air and to the aircraft is equal, but the amount of energy is not.
Energy absorbed by the brakes is equal to E=F*S, where F is braking force and S is distance traveled by the plane. If the brakes hold the plane, F=T, T being thrust, but S=0, and E=T*0=0. i.e. the plane won't move.
This assumes the brakes work perfectly and hold. Normally the brake will be released at half power. Past that, if the plane is lightly loaded, or has a lot of thrust, or if the runway is wet, the plane can start skidding, so you'll get movement and heating at the tires... And then this is what can happen.

Thanks for the answer!
So there is no friction and with that force absorbed through the clamping force the brakes are applying to the disks?

@Nico, think of it like this, the brakes aren't absorbing any energy from the engines, they absorb only the kinetic energy of the airplane.  If ground speed is zero there is no kinetic energy to absorb.

So I`ve asked ChatGPT about this as well, and it is very adamant that the brakes would heat up due to the static friction occuring between the brake pads and the wheel...
Do you by chance have any source on this?

Therac is correct.  Rub your hands together vigorously as hard as you can and see if your hands heat up.  Then push your hands together as hard as you can and try to move your hards together.  You feel the movement in your arms but the hands don't heat up due to friction.

@wbeard52 but wouldnt the muscles heat up in this case? causing the body to sweat after a certain amount of time?

@Nico I've done power assurance runs on CRJs where you run the engines up to TO thrust with the parking brake on. The brake temp indications are unaffected. It's as if the tires were just rigidly attached rubber feet like on the legs of a chair. There has to be movement between pads and discs.

Is the tire-to-ground friction sufficient to keep the airplane stopped with the engines at full power?  If the tires are skidding across the ground, the brakes will heat up a little via conduction from the heated tires.

I agree the muscles would heat up but that isn't due to friction.  The heat is due to the chemical release from the muscles pushing the hands together.... or something like that.

Never ask ChatGPT anything technical if you expect a correct answer.

@OrganicMarble or indeed anything at all.

of course depending on how good the brakes are the brakes may end up slipping, causing serious heating. You're assuming perfect braking action here, no slipping brakes, no skidding tyres, and no mechanical stresses on the brake mechanism.

@Mark Not always. For GA, sometimes enough, some not. For an airliner, on the very edge, hopefully-just-enough. Easily becomes "not enough" in cold or wet weather. For a jet fighter, not even close.

@OrganicMarble It's possible to use ChatGPT as a calculator, but only if you already know the answer. Just two days ago, it claimed the energy content of JP-1 was 0.43 MJ/kg and that it meant a turbine would need 3.5 kilograms of fuel to produce 100 kWh. Where did any of these figures come from? No one knows, it's buried deep in the weights of the language model.

@Therac it knows no math, science, or engineering. Just how to string words together in a plausible way. Very high powered BS generator, just what the world needs.

@Therac lol that's funny because according to Wikipedia: "Kerosene type BP Jet A-1, 43.15 MJ/kg" so it's off by 2 decimal points (assuming JP-1 and JET-A1 have similar calorafic value). Also, it's calculation is hot garbage; it would require around 8.4 kg to produce 100 kwh (840 kg if we use the calorafic value GPT came up with).

@AdityaSharma Yep. It's not just wrong; it's not even internally consistent, and in ways that make no sense. Maybe ChatGPT 5 with full Wolfram Alpha integration will be able to do good math. As of right now, it can be massaged to give the right answer, but you need to know the answer to massage it.

To be precise the engines do not generate energy, they convert chemical energy into kinetic energy, which can be measured as heat and thrust.
As long as the wheels of the aircraft are not rotating, no energy is transferred by/via the brakes, as there is no movement between brake pads and brake rotors that would cause friction create heat.
In any case, over time all of the energy  released by this process dissipates into the atmosphere, and eventually into space.

"Heat dissipates into the atmosphere".  Good.  Where does the thrust go?

Let it go Robert.

@RobertDiGiovanni Here is an entire discussion we had on that. Summary: with the brakes engaged and engines running, the earth and the air particles are gaining kinetic energy (both in opposite directions) - that's where the mechanical energy "produced" by the engine goes. Due to friction between earth and air, this kinetic energy will eventually be converted into heat.

@AdityaSharma thanks.  I won't worry about the friction of air slowing the planet's rotation.  Indeed, lucky for those at the equator, earth has a boundary layer!  But correctly, we realize fuel moves air, reaction (of varying efficiencies) provides propulsion force.  Locking brakes means plane + earth become a much more massive object.  To summarize, no friction heat will created unless the wheels rotate or skid.  Otherwise, it goes to earth.  Boring bookkeeping, but that is the science.  Our TOGA plane makes the world a bit warmer, and adds (or subtracts) from its rotation.

"it goes to earth"

@Jpe61 I mean, yes it does; as Bianfable said, "But a jet engine would, because it accelerates air backwards, so something else must be accelerated forwards (conservation of momentum)." so if the plane cannot accelerate, whatever it is anchored to (the earth) will have to. (not sure if your comment was meant to be sarcastic...)

@AdityaSharma Not all of it! The engine accelerates air, and the reaction force of accelerating the air accelerates the earth. It is not correct to say the energy goes to earth. Especially since most of the energy the jet "creates" is heat!

@Jpe61 yes, about 70% of fuel's chemical energy is converted into heat, while the remaining 30% is converted into mechanical energy. Of the 30%, some is acquired by air and the remaining by the earth (in the form of kinetic energy).


text:
Distance calculation for radius turn for Landing
Maybe if possible you could help from the below?
I'm at 140 knots following 354° inbound (CGS VOR) at Nice Airport LFMN.
From my heading 354°, I need to join 43°inbound for RWY 04L ( Not allowed ti make any turn prior to 5NM from CGS VOR). I would need to make a right turn of 49° to join the inboung 43°.
If I calculate my time for turning, would be (IAS speed/3° standard turn) = 140knots/3=16.3 seconds.
Then using the thumb rule, my bank angle should be (140/10)+7= 21°.
From these assumptions and the fact that I cannot turn before 5 NM from VOR, how do I know then when to turn to 43° inbound?
If I use the other formula IAS (approx to GS as close to RWY) squared / 10 , I get a radius of (140/60)2/10= 0.9 NM.
But this assumes a standard bank angle of 25-30° right?
Then in first assumption, my bank angle should be 21°. I'm confused there.
If I use a third formula, 1% of GS, I get a radius of  0.7 NM (1.4/2) correct?
Should I calculate the circumference of the circle and start turning from that circumference?
It would be great if you could help me understand this!

Are you asking this for some kind of simulation or theoretical exercise, or are you asking how the pilot actually does it? Because the pilot has no time to do these calculations in flight,  and does it in a much simpler (but less precise) way.

By the way, CGS is no longer a VOR/DME (it used to be), it's just a DME now, so your aircraft needs to be RNP capable for flying this approach.

What you are describing is the VPT A (Visual Maneuvering with Prescribed Tracks) for Rwy 04L or 04R into  Nice Côte d'Azur (LFMN) following the RNP A approach (which ends at MN04A):

Note that MN04A is the MAP (missed approach point) for the RNP approach with an MDA (minimum descent altitude) of 2000 ft. This means that you must be visual from that point on. The entire procedure shown above is flown entirely with visual references (you may use assistance to initially stay on the 354° track towards CGS, as the chart says, if equipment permits).
The point at which you would turn towards the runway is therefore determined by looking out of the window. There is no need to calculate anything, you just visually fly towards the runway. The only restriction is that you shouldn't deviate to the west on the initial segment towards CGS (to reduce noise in the city of Antibes). If you turn early, you can still correct to the left; if you turn late, you can still correct to the right. If you get the turn just right, you can intercept the PAPI for a visual descent towards the runway just after the turn.
If you misjudge the turn so bad that you can no longer safely correct your flight path to land, you would follow the Visual Balked Landing procedure (dashed lines) and then fly to NERAS (end of the missed approach procedure of the RNP A).
If the visibility is too poor to see the runway at MN04A or when there are clouds below 2000 ft, you would be cleared for the ILS or RNP Z approaches instead, which are both straight-in approaches overflying Antibes.

If you really want to calculate the point to start turning, consider the following sketch:

The first thing we need is the intersection point of the track 354° from MN04A and the extended runway centerline (I chose runway 04L here because that is the default runway for landing in Nice). I looked up the coordinates for the relevant points in my navigation database:

Point
Latitude
Longitude

LFMN 04L
43° 39' 9" N
7° 12' 17" E

LFMN 04R
43° 38' 48" N
7° 12' 9" E

MN04A
43° 33' 43" N
7° 9' 14" E

CGS
43° 38' 43" N
7° 8' 45" E

You can use the calculator on movable-type.co.uk to figure out the coordinates for the intersection point: 43° 36′ 28″ N, 007° 08′ 50″ E.
The next thing we need is the distance $D$ from the intersection point to CGS. The same page also has a calculator for that and gives $ D \approx 4.168 \, \text{km} \approx 2.251 \, \text{NM} $.
Now we calculate the turn radius $R$ using the formulas from this answer:
$$ R [\text{ft}] = \frac{V [\text{kt}]^2}{11.26 \tan \theta} \; , \qquad
\omega [^\circ/\text{s}] = \frac{1 \, 091 \tan \theta}{V [\text{kt}]} $$
When flying a standard turn at 3 degrees per second, we can solve the second equation for $\tan \theta$ and insert it into the first one:
$$ R [\text{ft}] = \frac{V [\text{kt}]^2}{11.26 \, \omega [^\circ/\text{s}] \, V [\text{kt}] / 1 \, 091} \approx 96.89 \frac{V [\text{kt}]}{\omega [^\circ/\text{s}]} $$
Assuming 140 kt TAS, this gives a turn radius of $ R \approx 4522 \, \mathrm{ft} \approx 0.7442 \, \mathrm{NM} $. The corresponding bank angle is then given by (source):
$$ b = 57.3^\circ \arctan \left( \frac{V [\text{kt}]}{362.1 \, \text{kt}} \right) \approx 21.14^\circ $$
The last thing missing is the distance $d$ before the intersection point, where the turn should be started. With a bit of geometry, one finds that this is given by
$$ d = R \tan \left( \frac{\alpha}{2} \right) $$
with our known $ \alpha = 49^\circ $. This results in $ d \approx 0.3392 \, \text{NM} $.
This gives the final answer for the distance from CGS where the turn should be started:
$$ D + d \approx 2.59 \, \text{NM} $$
Note that this entire calculation does not take wind into account!

thank you for your response.
It's for simulation purposes. I eeally would want to understand the how to calculate if there's no help from FMS or no chart. Would anyone be able to help me calculate that time to initiate the turn to 04L?

I cannit find words to thank you again! This is absolutely perfect. For the radius I think that the thumb rule of 1% of your IAS giving you the diameter works perfectlt as it gives you a turn radius of 0.7 NM, would you agree. Oh 1 more thing is the time for the turn, would you agree 49°/3= arpund 13 seconds?

Sorry forgot about the bank angle, do you agree it's (140/10)+7=21°?

Sorry not able to edit the comment so adding it separately. What about the bank angle? Do you agree it's 140/10 +7=21°?

And the time for the turn is also correct using 49°/3= 13 seconds?

@RO 49° divided by 3°/sec is ~16.3sec.

Sorry but I did try this on MFS2020 and the 16 seconds turn seems way too much, I definitely miss the 04L.I did start the turn at 2.6 CGS with a 21° bank angle. Did I miss something? Speed is 140 knots.

@RO I don't know how accurate that simulator is, so I cannot say what went wrong. Maybe different TAS? Maybe wind? I've flown that approach many times and I never had issues with simply looking out of the window and seeing the airport.

I think I'm good now today I've tried it several times and 16 seconds work! Thanks again for your help


text:
Downwash vs Finite-Wing Downwash
Is there a distinction to be made between downwash created due to lift generation of an infinite wing or airfoil and the "induced downwash" created due to a finite wing. I've been thinking of this additional downwash due to lift as an increment to the existing downwash. Is this a helpful way to think about it?

This answer should help.


text:
Why aren't we using forward swept wings in commercial airliners?
What are the advantages and disadvantages of doing so.
Is there any practical problem in execution?
Provide some experiments for examples.

Does this answer your question?

Related: https://aviation.meta.stackexchange.com/q/4306/7532


text:
Mode-S, 1090ES and ADS-B
I have an old 2008 version of a Becker BXP6403 Mode S transponder. It is a Class 2 level 2ES device. As it is an early version it says it does not support ADS-B out.
Becker have told me that it has "Mode”S” functionality with extended squitter. There is an interface for an Avionics Data Link processor and a data transfer interface protocol document available. It does not accept GPS NMEA data (so they say it does not transmit GPS position).
The data interface is fairly basic but it does has a command to be able to set GICB (Ground initiated Comm-B) registers. I have assumed that these are the equivalent to the 1090ES BDS registers.
I can successfully establish a connection to the transponder and send  messages to the GICB registers - I get a reply back from the transponder with an updated message counter so I´m assuming they have been correctly received however there is no way to check.
I know the transponder is "squitting" as I attached a neon to the antenna and I see it regularly flashing. I don´t know what is being sent though.
I have a FlightAware PiAware system running and I see aircraft that are near to me but I don´t see any messages from my transponder.  I know the transponder can be seen by ATC as I checked with them whilst I was flying and they could see me and my altitude (although this could have been from Mode-C I guess). I didn´t confirm if they could see my ICAO code (maybe I should re-check).
I´ve also set up a (separate) ModeSDeco2 installation but this doesn´t see messages from me either.
So my question(s) are:

If the transponder is Mode-S and is squitting data then why can't my PiAware or ModeSDeco systems see me?
Is there more of a difference between Mode-S and ADS-B than there appears to be?
Is my assumption correct about the GICB and BDS registers?

It seems I´m missing something but everything I seem to find about Mode-S refers to it as squitting ADS-B messages.

Correct, this is why I am trying to get it functioning and why I was expecting to see my own transponder squitting on PiAware and ModeSDeco - but I don´t.


text:
Do you need a working VOR ground facility to fly a VOR approach if you have an FAA approved WAAS GPS? (e.g. G1000)
since TSO C145 and C146 are considered stand-alone units (you don't need an actual VOR receiver in a/c to fly the VOR approach or to fly on V airways), However, do the ground-based navaids (VOR) still have to be operational in order to fly these approaches, procedures and on these airways?

Yes.  The underlying NAVAID must be operational and used to fly a VOR approach.  Please reference AIM 1-2-3(c)(note #5).

C. Uses of Suitable RNAV Systems. Subject to the operating requirements, operators may use a suitable RNAV system in the following ways.

Determine aircraft position relative to, or distance from a VOR (see NOTE 6 below), TACAN, NDB, compass locator, DME fix; or a named fix defined by a VOR radial, TACAN course, NDB bearing, or compass locator bearing intersecting a VOR or localizer course.
Navigate to or from a VOR, TACAN, NDB, or compass locator.
Hold over a VOR, TACAN, NDB, compass locator, or DME fix.
Fly an arc based upon DME.

NOTE-

The allowances described in this section apply even when a facility is identified as required on a procedure (for example, “Note ADF required”).
These operations do not include lateral navigation on localizer-based courses (including localizer back-course guidance) without reference to raw localizer data.
Unless otherwise specified, a suitable RNAV system cannot be used for navigation on procedures that are identified as not authorized (“NA”) without exception by a NOTAM. For example, an operator may not use a RNAV system to navigate on a procedure affected by an expired or unsatisfactory flight inspection, or a procedure that is based upon a recently decommissioned NAVAID.
Pilots may not substitute for the NAVAID (for example, a VOR or NDB) providing lateral guidance for the final approach segment. This restriction does not refer to instrument approach procedures with “or GPS” in the title when using GPS or WAAS. These allowances do not apply to procedures that are identified as not authorized (NA) without exception by a NOTAM, as other conditions may still exist and result in a procedure not being available. For example, these allowances do not apply to a procedure associated with an expired or unsatisfactory flight inspection, or is based upon a recently decommissioned NAVAID.
Use of a suitable RNAV system as a means to navigate on the final approach segment of an instrument approach procedure based on a VOR, TACAN or NDB signal, is allowable. The underlying NAVAID must be operational and the NAVAID monitored for final segment course alignment.
For the purpose of paragraph c, “VOR” includes VOR, VOR/DME, and VORTAC facilities and “compass locator” includes locator outer marker and locator middle marker.


text:
LFLL LSE VOR 35L Visual Approach
Was wondering whether someone could help me understand how to perform a visual approach (not using the VOR) specifically if I look at RWY 35L at LFLL. Airport elevation is 821 ft.
It's a 3° GS approach if you look at charts for the LSE VOR. Therefore I was thinking why would the 3x rule not work for visual approach landing?
I mean charts say that I should be  at 2740 ft at 8 DME, 2420 ft at 7 DME, 2100 ft at 6 DME, 1790 ft at 5 DME, 1470 ft at 5 DME.
These do not seem to align to ×3 rule which we could use to see if we're on profile when we start our descent.
If I use this rule I should be at 1800 ft at 6 DME, 1500ft at 5 DME and 1200 ft at 4DME correct? What am I missing here?
Flying at S spoed so rate of descent should equate to roughly a 3° descent if I use a vertical speed of GSx5.
What is the technique then for a visual approach if we can't use that x3 rule?
The VOR seems not be be offset and is situated near TDZE it seems so was wondering why we can't use x3 rule?

Your difference at 5 dme is just 30 feet!

The runway end is 2.1 DME from the VOR. Are you taking that into your equation?

I do not intend to answer, but your question may be similar in some ways to the question I asked: https://aviation.stackexchange.com/q/97583/65367

Why do you care about the vertical profile of the VOR approach if you intend to fly a visual approach?

What is the technique then for a visual approach if we can't use that x3 rule?

To be blunt, look out the window.
Nothing says you can't use the x3 rule, and nothing says you can't occasionally reference the stepdown altitudes in a published instrument approach procedure.  (they aren't that different - 30' at 5 miles is insignificant...)  Neither is a bad idea, but there is no requirement to hit any altitudes so you shouldn't fixate on either one.
ICAO:

An approach by an IFR flight when either part or all of an instrument approach procedure is not completed and the approach is executed in visual reference to terrain.

FAA:

A visual approach is an ATC authorization for an aircraft on an IFR flight plan to proceed visually and clear of clouds to the airport of intended landing. A visual approach is not a standard instrument approach procedure and has no missed approach segment.

While you remain on an IFR clearance, you should fly the approach as if you were VFR.
So again, look out the window - and make sure your approach path is clear of terrain, clouds, and the descent angle is appropriate.

Agree with the sentiment, but for the sake of completeness the FAA definition of a visual approach probably doesn't carry a lot of weight in Lyon, France. The FAA's definition is similar to ICAO's definition — An approach by an IFR flight when either part or all of an instrument approach procedure is not completed and the approach is executed in visual reference to terrain — and the crux of the matter, "visual reference," is found in both definitions.

@randomhead, good point.  To be honest I didn't look at the approach plate for location.  The definitions are similar enough, but I added the ICAO in the interest of being thorough and complete.  Thanks.